Throughout the world there is an ever-increasing interest in thermal insulation for buildings. One form of such insulation involves securing foamed-plastic insulating slabs or plates, such as those of foamed polystyrene, to outer surfaces of walls to be insulated. This is conveniently accomplished with mineral plaster or mortar which ordinarily contains at least 5 percent by weight of plastic resin. The outside of the slabs is then covered with a similar plaster or mortar which is suitably reinforced by, e.g., an embedded glass-fiber web, animal hair, cocoa, sisal and/or synthetic fibers.
Some difficulty is encountered because the hard-foam slabs are subject to a material degree of shrinkage over an extended period of time, i.e., as residual foaming agent and solvent emanate therefrom. The resulting contraction is more than and in excess of the maximum possible thermal contraction which, in turn, differs from the thermal contraction or expansion of the covering plaster or mortar. Both the shrinkage and the differences in thermal coefficients increase the expectation of cracks and subsequent deterioration of the covering plaster or mortar.
In an attempt to minimize this problem, such slabs or plates are usually stored before use for an extended period of time, i.e., until a residual shrinkage of not more than 0.2 percent (2 millimeter per meter) is expected. Even with the use of slabs or plates having a thickness between one inch (2.54 cm) and two inches (5.08 cm), cracking or blistering of the outer plaster or mortar could not always be prevented. The problem increased with increased thicknesses of the hard-foam slab or plate.
By using plaster or mortar with a high resin content, the resulting plaster or mortar is elastic and thus has less tendency to crack. Unfortunately, the increase in resin content also makes the plaster or mortar soft. Whereas elasticity is a welcome characteristic, softness is not. Softness is actually highly undesirable for the outer surface of a building. Moreover, plastic resins or similar adhesives increase the water-vapor diffusion-resistance factor of plaster or mortar. When such factor is too high, moisture accumulates in the hard-foam slabs or plates, and this eventually leads to their destruction. However, addition of some resin is highly desirable since pure mineral plaster or mortar does not sufficiently adhere to the surface of hard-foam slabs or plates.